That bunnit's got a vicious streak a mile wide, it's a killer!  He's got huge, sharp...and he can leap about...LOOK AT THE BONES!

From what I've seen all levels stay the same but the's a trade off you can decide to stay the same level if you like or you can rest at an Inn and reduce your level for which you get bonus exp points which are used to unlock new traits and features on characters and blades like hidden affinities and additional accessory slots and such.

I don't understand this new game +.

That means that it make you start from scratch?

You are beginning the same story with high level and all your blades? What's the points?

Does it destroy your previous file save?

1. Story, side quests, Heart to Hearts and locations get restarted.

2. Yes, but you can decrease the level, and you also start with all your Blades. The appeal here is that you can  try to complete the game at lower levels, or maybe complete the whole story at level 99 and your overpowered Blades, and you can change such level at will by just sleeping at the inns; it's up to you. There's already someone who defeated the level 130 superboss at level 1, which is not surprising when you know that overpowered Blades are more important than the level of the Drivers itself. The NG+ function also allows you to bond with all the known Torna members (minus Jin and Malos), including their Blades who were supossed to pass away in the story.

Another significant thing is that you can add a third accessory slot to the Drivers. Basically, NG+ is "let's break the game for fun" edition. 

3. Yes, but again, you start with everything you had in the original file, with the exception of a couple of things here and there. 

If you have purchased the season pass that blade (late game players will know who) now has a new special added.

Do the Blade cores transfer to NG+, I beat the game over 200 hours of play time ago and have mainly been grinding Blade affinities while burning through rare and legendary cores trying to get KOS-MOS, she and Dahilia are the only two rare Blades I don't have but I REALLY want KOS-MOS.

I'm starting Xenoblade Chronicles New Game + right now, just as I go to sleep.

That new game plus is strange but cool. 

There are 5 people on the Xenoblade team that have been working together since Chrono Trigger - the first game to use New Game+

Although only Tetsuya Takahashi, Makoto Shimamoto, and Yasunori Mitsuda worked on Xenoblade 2 from those 5. Yasuyuki Honne and Tadahiro Usuda on Breath of the Wild.

Makoto Shimamoto also moved onto Breath of the Wild at some point (or the other way around, she moved onto XC2 after Zelda) as she has a credit on Breath of the Wild for game design. For the record, she is credited with a lot as a designer:
Chrono Trigger
Radical Dreamers (Writer)
Xenogears
Xenosaga 1-3
Soma Bringer
Xenoblade Chronicles 1, X, and 2
Legend of Zelda: Breath of the Wild
Her job on Chrono Trigger was interesting, the scripted battles where the enemies jump out into position and such, she did a lot of that content.